It's the public service. It is the Prime Minister's department, but it is filled with public servants who generally have worked elsewhere to have a broader experience. They spend some time at the PCO, and then typically it is a path to more senior jobs in the public service.
I was the deputy secretary of operations in the PCO before I became the deputy minister of justice, for example. Before that, I was the assistant secretary of economic and regional development policy, starting under the Conservatives at the end of the Mulroney and Kim Campbell regimes.